Learning Objectives

After studying this topic, you should be able to:

• Define Biogeochemical Cycles.

• Classify biogeochemical cycles into Gaseous and Sedimentary cycles.

• Explain the Carbon, Nitrogen, Phosphorus, and Sulfur cycles.

• Identify the microorganisms involved in the Nitrogen Cycle.

• Differentiate between major biogeochemical cycles.

Biogeochemical Cycles

Definition

A Biogeochemical Cycle is the cyclic movement of chemical elements and nutrients between the biotic (living) and abiotic (air, water, soil, rocks) components of the environment.

Meaning of the Term

• Bio → Living organisms

• Geo → Earth (soil, rocks, water, atmosphere)

• Chemical → Elements such as Carbon, Nitrogen, Phosphorus, Sulfur

Importance

• Recycles essential nutrients.

• Maintains ecological balance.

• Ensures continuous availability of nutrients.

• Supports life on Earth.

Basic Components of a Biogeochemical Cycle

1. Reservoir (Pool)

The main storage place of a nutrient.

Examples

• Atmosphere → Nitrogen

• Atmosphere & Oceans → Carbon

• Rocks → Phosphorus

• Rocks & Soil → Sulfur

2. Flux (Pathway)

The route or process through which nutrients move from one reservoir to another.

Examples

• Photosynthesis

• Respiration

• Weathering

• Decomposition

• Nitrogen fixation

Classification of Biogeochemical Cycles

Biogeochemical cycles are classified into two major types.

1. Gaseous Cycles

Definition

Cycles in which the primary reservoir is the atmosphere or oceans.

Characteristics

• Fast cycling

• Short turnover time

• Atmospheric reservoir

Examples

• Carbon Cycle

• Nitrogen Cycle

• Oxygen Cycle

• Water (Hydrological) Cycle

2. Sedimentary Cycles

Definition

Cycles in which the primary reservoir is the Earth's crust (rocks and soil).

Characteristics

• Very slow

• Long turnover time

• Geological processes involved

Examples

• Phosphorus Cycle

• Sulfur Cycle

Comparison: Gaseous vs Sedimentary Cycles

Carbon Cycle

Reservoir

• Atmosphere (CO₂)

• Oceans

Sources of Carbon Dioxide

Carbon returns to the atmosphere through:

• Respiration

• Aerobic decomposition

• Combustion of fossil fuels

• Volcanic activity

Carbon Sinks

Carbon is removed from the atmosphere by:

• Photosynthesis

• Ocean absorption

• Marine phytoplankton

Flowchart

Atmospheric CO₂
        ↓
Photosynthesis
        ↓
Green Plants
        ↓
Animals
        ↓
Respiration & Decomposition
        ↓
Atmospheric CO₂

Nitrogen Cycle

Reservoir

• Atmosphere (78% Nitrogen gas (N₂))

Important Point

Plants cannot absorb atmospheric nitrogen (N₂) directly.

They absorb Nitrate (NO₃⁻).

Five Major Steps of Nitrogen Cycle

1. Nitrogen Fixation

Definition

Conversion of atmospheric Nitrogen (N₂) into Ammonia (NH₃).

Nitrogen-Fixing Bacteria

2. Nitrification

Definition

Conversion of:

Ammonia (NH₃)
      ↓
Nitrite (NO₂⁻)
      ↓
Nitrate (NO₃⁻)

Bacteria Involved

3. Assimilation

Definition

Plants absorb Nitrate (NO₃⁻) from the soil and convert it into proteins and other organic compounds.

4. Ammonification

Definition

Decomposers convert dead plants, animals, and organic wastes into Ammonia (NH₃).

5. Denitrification

Definition

Conversion of Nitrate (NO₃⁻) back into atmospheric Nitrogen (N₂).

Bacteria

• Pseudomonas

Flowchart: Nitrogen Cycle

Atmospheric Nitrogen (N₂)
           │
           ▼
Nitrogen Fixation
(Rhizobium, Azotobacter, Clostridium)
           │
           ▼
Ammonia (NH₃)
           │
           ▼
Nitrosomonas / Nitrococcus
           │
           ▼
Nitrite (NO₂⁻)
           │
           ▼
Nitrobacter
           │
           ▼
Nitrate (NO₃⁻)
           │
           ▼
Assimilation by Plants
           │
           ▼
Animals
           │
           ▼
Death & Wastes
           │
           ▼
Ammonification
           │
           ▼
Ammonia
           │
           ▼
Pseudomonas
(Denitrification)
           │
           ▼
Atmospheric Nitrogen (N₂)

Phosphorus Cycle

Reservoir

• Phosphate rocks

• Earth's crust

Characteristics

• Strictly sedimentary cycle

• No gaseous phase

• Very slow cycle

Process

Phosphate Rocks
        ↓
Weathering
        ↓
Soil
        ↓
Plants
        ↓
Animals
        ↓
Decomposition
        ↓
Soil

Sulfur Cycle

Reservoir

• Sulfide and sulfate rocks

• Soil

Characteristics

• Mainly sedimentary

• Has a gaseous phase

Gaseous Forms

• Sulfur dioxide (SO₂)

• Hydrogen sulfide (H₂S)

Sources

• Volcanic eruptions

• Burning of fossil fuels

Return to Earth

• Acid rain (H₂SO₄)

Comparison Tables

Carbon Cycle vs Nitrogen Cycle

Phosphorus Cycle vs Sulfur Cycle

Nitrogen Cycle Microorganisms

Important Definitions

Biogeochemical Cycle

Movement of nutrients between living and non-living components.

Reservoir

Main storage location of a nutrient.

Flux

Movement pathway of nutrients.

Assimilation

Absorption of inorganic nutrients by plants.

Mineralization

Conversion of organic matter into inorganic nutrients by decomposers.

Aerobic Decomposition

Breakdown of organic matter in the presence of oxygen, producing CO₂.

Anaerobic Decomposition

Breakdown of organic matter in the absence of oxygen, producing CH₄ (Methane).

Frequently Confused Concepts

Nitrogen Fixation vs Nitrification

Nitrification vs Denitrification

Phosphorus vs Sulfur Cycle

Energy Flow vs Biogeochemical Cycle

Examination Focus (High Probability MCQs)

Frequently Asked Questions

• Definition of Biogeochemical Cycle.

• Classification of cycles.

• Reservoir of Carbon, Nitrogen, Phosphorus, and Sulfur.

• Steps of Nitrogen Cycle.

• Bacteria involved in:

  • Nitrogen fixation

  • Nitrification

  • Denitrification

• Which nutrient do plants absorb?

• Which cycle has no gaseous phase?

• Which cycle has a gaseous component despite being sedimentary?

Common Mistakes

❌ Plants absorb atmospheric nitrogen (N₂).

✔ Plants absorb Nitrate (NO₃⁻).

❌ Nitrobacter converts ammonia to nitrite.

✔ Nitrosomonas/Nitrococcus convert NH₃ → NO₂⁻.

✔ Nitrobacter converts NO₂⁻ → NO₃⁻.

❌ Sulfur is a completely sedimentary cycle.

✔ Sulfur has a gaseous phase (SO₂, H₂S).

❌ Phosphorus has an atmospheric phase.

✔ Phosphorus has no gaseous phase.

Memory Tricks

Classification

CNOW → Air

• C → Carbon

• N → Nitrogen

• O → Oxygen

• W → Water

(Gaseous Cycles)

PS → Soil

• P → Phosphorus

• S → Sulfur

(Sedimentary Cycles)

Nitrogen Cycle Bacteria

Rhizo Fixes → Nitro Builds → Pseudo Returns

• Rhizobium → Nitrogen Fixation

• Nitrosomonas → NH₃ → NO₂⁻

• Nitrobacter → NO₂⁻ → NO₃⁻

• Pseudomonas → Denitrification

Nitrification Sequence

A → Ni → Na

• A → Ammonia

• Ni → Nitrite

• Na → Nitrate

Quick Revision

One-Line Revision

• Biogeochemical Cycles → Recycle nutrients between living and non-living components.

• Gaseous Cycles → Carbon, Nitrogen, Oxygen, Water.

• Sedimentary Cycles → Phosphorus and Sulfur.

• Carbon Cycle → CO₂ fixed by photosynthesis and released by respiration, decomposition, and combustion.

• Nitrogen Cycle → Fixation → Nitrification → Assimilation → Ammonification → Denitrification.

• Plants absorb nitrate (NO₃⁻), not atmospheric N₂.

• Rhizobium → Nitrogen fixation.

• Nitrosomonas → NH₃ → NO₂⁻.

• Nitrobacter → NO₂⁻ → NO₃⁻.

• Pseudomonas → Denitrification.

• Phosphorus Cycle → Strictly sedimentary with no gaseous phase.

• Sulfur Cycle → Sedimentary cycle with a gaseous phase (SO₂, H₂S).